This invention relates to worm gears, and is more particularly directed to improvements in the shape of the threads of worm gear for the purpose of, for example only, avoiding clamping of the threads thereof when the gears are subject to inclined forces.
A worm gear of known type, such as disclosed, for example, in U.S. Pat. No. 4,679,458, is advantageously employed for the translation of rotational movements to linear movements. By employing a correspondingly larger thread pitch, the function of this gear can be reversed, i.e. to translate linear movements to rotational movements. Instead of employing only a single thread, it is alternatively possible to employ multiple threads, so that with the same load carrying capacity is attained with a higher pitch. The threads have trapezoidal cross sections, whereby relatively large surface thread flanks are provided, thereby attaining a higher load carrying capacity. Under axial loading it is attampted as much as possible for the loading to be distributed over the entire thread length. In other words, the threads of the spindle and of the nut must be made with extremely low tolerance. In order to avoid the necessity of manufacturing the threads with all of the thread dimensions having extreme accuracy, and to avoid the necessity of matching of individual threads, the spindle is provided in many cases with minimum axial play with respect to the nut, whereby a minimum gap results between the unloaded thread flanks under pure axial force.
The gap occurs on the other thread flanks of the outer and inner threads under reverse axial loading. In the case of radial loading on the other hand both thread flanks of the inner threads are in contact with the corresponding flanks of the outer threads at the corresponding circumferential positions of the threads. The known arrangement is so constructed that in this case for example a contact occurs in the region of the loaded thread flanks between the thread crown surface of the inner thread and the groove bottoms of the outer threads.
In complex force distribution the case can arise wherein the nut becomes inclined or is tipped with respect to the spindle. This limiting case disadvantageously affects the function of the worm gear, especially if rotation of the spindle due to the linear movement of the nut is desired or if rotation of tho nut responsive to linear movement of the spindle is desired. Due to the inclined position of the nut and spindle, the support surface is reduced to a minimum and it remains as only a disappearingly small line contact, or even a point contact, on two radial or axial opposed transition regions of the thread flanks to the thread crown surfaces of the outer thread on the threaded spindle with the corresponding partial surface of the inner threads of the nut. In many cases this inclined position results in clamping with small deformation of the contact surfaces. A relative rotation between the nut and spindle is possible in this case only with difficulty. In the above example the translation of linear of rotational movement results in self-locking, which no longer permits movement. The inclined position must be first corrected in order to insure proper functioning of the worm gear.
In the known arrangement the threads become supported over the inclined surfaces on the groove bottoms in response to tipping forces. Clamping is thereby almost avoided. In the sense of smooth functioning of the worm gear it is not necessary that the threaded surfaces remain in contact. Contact can also occur here; it must only be taken into consideration, however, that none of the force components resulting from the inclined force occur in the region of the thread flanks. This is not always possible, however, due to tolerance errors resulting from manufacturing requirements.